The invention relates to an internal combustion engine with an exhaust turbocharger including a turbine and a compressor and an exhaust-gas recirculation device, the turbine having a plurality of flow passages for feeding exhaust gas to the turbine rotor from two separate exhaust lines connected to different cylinders of the internal combustion engine.
Such an internal combustion engine is described in the printed publication DE 198 57 234 C2. The internal combustion engine is equipped with an exhaust turbocharger, the exhaust turbine of which has two spiral flow passages, which are supplied with exhaust gas via respective exhaust lines, each connected to a respective bank of cylinders of the internal combustion engine. The two flow passages have cross sections of different sizes, causing a higher exhaust backpressure in the line system leading to a smaller flow passage. This line system is connected to an exhaust-gas recirculation device, by means of which an adjustable mass flow of exhaust gas can be transferred from the exhaust section to the intake duct to reduce exhaust emissions. Owing to the higher pressure level in the smaller flow passage, the exhaust gas can be recirculated in a relatively large operating range of the internal combustion engine.
The determination of the size ratio of the two flow passages relative to one another is of decisive importance for the exhaust-gas and consumption behavior of the internal combustion engine.
It is the object of the present invention to provide an internal combustion engine with an exhaust gas turbocharger, which has low pollutant emissions and low fuel consumption.
In an internal combustion engine including an exhaust turbocharger and an exhaust-gas recirculation device, the turbocharger comprises an exhaust turbine and a compressor wherein the exhaust turbine has a plurality of inlet flow passages to which exhaust gas can be supplied via separate exhaust lines connected to some of the cylinders of the internal combustion engine. In total, three flow passages are provided in the exhaust turbine, of which two communicate with the exhaust-gas recirculation device, and one of these flow passages includes an adjustable throttling member for regulating the mass flow of exhaust gas to be recirculated.
By means of the adjustment of the throttling member, the mass flow of exhaust gas can be directed to the relevant flow passage to be regulated and, if appropriate, shut off completely, with the result that, where these two flow passages are supplied via a common exhaust line, the entire mass flow of exhaust gas in this line is directed into the flow passage without a throttling member, thereby allowing an increased pressure level to be set because of the overall smaller flow cross-section. As as result, exhaust-gas recirculation is possible in a larger operating range, in particular at low engine and charger speeds.
In an expedient embodiment, the throttling member, which is arranged either in the feed line to the flow passage or, alternatively, in the turbine casing, is part of a blow-off device, via which an adjustable portion of the mass flow of exhaust gas can be blown off, bypassing the exhaust turbine; in this case, the throttling member is preferably in the form of a three-way valve. Upon severe throttling, the exhaust-gas flow is forced into the flow passage without a throttling member, thereby providing therein for a higher pressure level. With blow-off, an impermissible excess pressure in the exhaust line can be avoided, in order to prevent over-speeding of the turbocharger.
In another expedient embodiment, exhaust gas is supplied to both flow passages involved in exhaust-gas recirculation via a common exhaust line, whereas the third flow passage, which is not involved in exhaust-gas recirculation, is connected to the internal combustion engine by a separate exhaust line. The two exhaust lines are advantageously connected to different banks of cylinders of the internal combustion engine, via which the exhaust gas from respective groups of cylinders of the internal combustion engine can be fed to the respective flow passages. Both symmetrical and asymmetrical division of the number of cylinders connected to each exhaust line is conceivable, the exhaust gas from the larger number of cylinders preferably being fed to the third flow passage, that is not involved in exhaust-gas recirculation.
The exhaust turbine is expediently equipped with variable turbine geometry, by means of which the flow inlet cross section of at least one flow passage leading to the turbine rotor can be set in a variable manner. The variable turbine geometryxe2x80x94e.g. a guide vane system that can be displaced axially into the flow inlet cross-section or a guide vane system with adjustable guide vanesxe2x80x94is advantageously situated in the flow inlet cross section of the flow passage that is not involved in exhaust-gas recirculation. In addition or as an alternative, the variable turbine geometry can also be arranged in the flow inlet cross-section of the flow passage that is involved in exhaust-gas recirculation and which also includes the throttling member. If appropriate, this variable turbine geometry can be adjusted independently of the flow passage that is separate from exhaust-gas recirculation.
The position of the variable turbine geometry can be used as an additional adjustment variable for the optimization of the powered mode and also of the engine-braking mode. Further possibilities of adjustment are obtained by adjusting the throttling member and, if appropriate, a recirculation valve located in a recirculation line of the exhaust-gas recirculation device.
The two flow passages involved in exhaust-gas recirculation are expediently smaller than the third flow passage, which is not involved in exhaust-gas recirculation. In establishing the size ratio, it is possible to use a turbine throughput parameter, which can be determined from the mass flow of exhaust gas, the temperature and the pressure in each flow passage. The sum of the turbine throughput parameters through the two smaller flow passages involved in exhaust-gas recirculation is expediently in a range of between 70% and 120% of the corresponding turbine throughput parameter for the larger flow passage, which is not involved in exhaust-gas recirculation. The relatively small flow passages, which communicate with the exhaust-gas recirculation device, allow a higher exhaust backpressure than the larger flow passage, which benefits exhaust-gas recirculation. It may be advantageous to make the flow passages involved in exhaust-gas recirculation smaller, when taken together, than the third flow passage, making the sum of the turbine throughput parameters of the two smaller flow passages smaller (less than 100%) than the turbine throughput parameter of the larger third flow passage.
Given the ratio of the turbine throughput parameters to one another, it is possiblexe2x80x94if necessary after establishing absolute values for the turbine throughput parameters, e.g. as a function of the displacement of the internal combustion enginexe2x80x94to deduce the actual geometric dimensioning of each flow passage on the basis of an empirically or, if appropriate, analytically determined relationship. From such a relationship, it is possible, in particular, to determine the spiral cross section of each spiral flow passage and the radial distance between the central axis in the flow inlet of the spiral cross section and the axis of rotation of the turbine rotor. In principle, these geometric variables are sufficient for the design configuration of the flow passage.
In another advantageous embodiment, the size ratio of the two smaller flow passages, which are involved in exhaust-gas recirculation, can be determined from the ratio of the relevant turbine throughput parameters. The value for the throughput parameter of the flow passage without a throttling member is advantageously in a range of values between 40% and 150% of the turbine throughput parameter for the flow passage with a throttling member, values of less than 100%xe2x80x94flow passage without a throttling member smaller than flow passage with throttling memberxe2x80x94being particularly advantageous since the smaller of the two flow passages allows a higher pressure build-up. In conjunction with the design rule mentioned above, all three flow passages can be dimensioned, if necessary after establishing an absolute amount, e.g. for the total turbine throughput parameter, which is formed by summing all three individual turbine throughput parameters, as a function of the displacement of the internal combustion engine or some other characteristic value.